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International audience; Here, we report the complete genome sequence of Bradyrhizobium sp. strain ORS3257, which forms efficient symbioses with cowpea, peanut, or groundnut. These genomic data will be useful to identify genes associated with symbiotic performance and host compatibility on several legumes, including Aeschynomene species , with which a Nod-independent type III secretion system (T3SS)-dependent sym-biosis can be established.

Here, we report the complete genome sequence of Bradyrhizobium sp. strain ORS3257, which forms efficient symbioses with cowpea, peanut, or groundnut., Here, we report the complete genome sequence of Bradyrhizobium sp. strain ORS3257, which forms efficient symbioses with cowpea, peanut, or groundnut. These genomic data will be useful to identify genes associated with symbiotic performance and host compatibility on several legumes, including Aeschynomene species, with which a Nod-independent type III secretion system (T3SS)-dependent symbiosis can be established.

Arbuscular mycorrhizal symbiosis is an important plant root–fungal partnership/interaction that affects the growth response of crops. We have investigated the molecular diversity of arbuscular mycorrhizal fungi (AMF) colonizing cowpea roots and the associated rhizosphere soil to test the hypothesis that community diversity in rhizosphere soil is similar to that in cowpea (Vigna unguiculata) roots. Cowpea plants were grown in farmers’ fields located in seven agro-ecological zones of Benin, and soil and root samples were collected. The molecular diversity of the AMF in these samples was assessed after amplification of the large ribosomal subunit of DNA extracted from the soil and the root samples. At fruition, the frequency of mycorrhizal infection was unaffected by the agro-ecological zone, but there were significant differences in the intensity of AMF colonization among the zones. Multiple regression analysis showed that the main factor affecting mycorrhizal frequency at flowering was available phosphorus. Phylogenetic analysis revealed 25 operational taxonomic units belonging to two fungal families (Glomeraceae and Gigasporaceae). The diversity of AMF colonizing roots of cowpea in Benin was high and fairly similar to that in the rhizosphere soil but with a prevalence of the Glomeraceae. Despite the absence of strict host specificity in mycorrhizal symbiosis, there was a preferential association between some AMF species and cowpea cultivar IT96D-610.

The response of cowpea (Vigna unguiculata (Walp. (L.)) to inoculation with symbiotic microorganisms (arbuscular mycorrhizal fungus and rhizobium) was investigated on cultivated vertisoils (known locally as Dek) and arenosoils (known locally as Dior) in three geographic sites in Senegal. On the basis of soil physico-chemical characteristics, of most probable number (MPN) of indigenous rhizobia and of yields, a hierarchical analysis showed that the sites were grouped in three clusters. Some soils initially considered as Dek were grouped with Dior, so that all of Dior soils were grouped in the same cluster. No significant increasing in shoot biomass or yield was observed when cowpea was inoculated with fungi alone, whatever soil type. However, a significant or noticeable improvement in cowpea seeds production was obtained by rhizobium inoculation. It appeared that cowpea response to inoculation depended on geographical site and were often observed in sandy arenosoils (Dior type) or in loamy sand vertisoils (Dek type) with at least 15mgkg -1 of available

International audience; In a context of global change, alterations in the water cycle may impact the structure and function of terrestrial and aquatic ecosystems. Wetlands are particularly at risk because hydrological regime has a major influence on microbially mediated biogeochemical processes in sediments. While the influence of water availability on wetland biogeochemical processes has been comprehensively studied, the influence of hydrological regime on microbial community structure has been overlooked. We tested for the effect of hydrological regime on the structure and functions of microbial communities by comparing sediments collected at multiple sites in the Ain departement (Eastern France). Each site consisted of two plots, one permanently and one seasonally inundated. At the time of sampling, all plots were continuously inundated for more than 6 months but still harboured distinct bacterial communities. This change in community structure was not associated with marked modifications in the rates of microbial activities involved in the C and N cycles. These results suggest that the observed structural change could be related to bacterial taxa responding to the environmental variations associated with different hydrological regimes, but not strongly associated with the biogeochemical processes monitored here.

Tilemsi rock phosphate (TRP) of Mali is one of the most promising rock phosphate in West Africa for soil fertilization, but it is little used because of its insoluble form. The main objective of this study is to investigate TRP effects on rhizobia associated with the multipurpose leguminous tree Sesbania sesban grown on a sandy soil, poor in phosphorus and not sterilised. The experiment included treatments with and without TRP and was conducted during 105 days. At the end, 114 nodules have been collected and analysed by PCR/RFLP of 16S-23S intergenic spacer. Sixteen different RFLP profiles corresponding to different genomic groups of rhizobia have been detected. Five were dominant and present in both treatments. Five groups appear only in treatments without TRP whereas the six others are only in nodules of plants with TRP, suggesting a different capacity of natural phosphates solubilization by these strains.

The present study aimed at assessing the recovery of phototrophic and heterotrophic biofilm communities after a decrease in copper exposure. An original experiment was designed to evaluate the possible influence of non-exposed (i.e. pristine) communities (e.g. via immigration processes) in recovery dynamics. Laboratory channels were used to study the structural and functional changes in microbial communities after a 4-week Cu exposure period in the presence and absence of pristine biofilms. When pristine biofilms were present, phototrophic communities recovered within 6 weeks, both in terms of biomass, structure and photosynthetic activity. Recovery processes were also detected using the PICT approach. In contrast, in the absence of pristine communities, all of the Cu-induced changes recorded in the phototrophic communities remained throughout the recovery period. Regardless of the presence or absence of pristine biofilms, the decrease in Cu exposure did not abolish Cu-induced changes in bacterial community structure, whereas functional recovery (based on beta-glucosidase activity) was complete in both recovery contexts. These results revealed that microbial community response to a decrease in Cu exposure differs between phototrophic and heterotrophic communities. The presence of pristine communities greatly influences the structural and functional recovery of phototrophic communities, suggesting an important role of microbial immigration processes, but have far less influence on the recovery trajectory of heterotrophic communities.

Aims: To assess the phenotypic, symbiotic and genotypic diversity scope of Mesorhizobium spp. strains associated with Acacia seyal (Del.) isolated from different agro-ecological zones in Senegal, and uses of susceptible microbial inoculum in a reafforestation process. Methods and Results: A polyphasic approach including phenotypic and genotypic techniques was used to study the diversity and their relationships with other biovars and species of rhizobia. The geographical origins of the strains have limited effect on their phylogenetic and phenotypic classification. Nodulation tests indicated promiscuity of the strains studied, because they were capable of nodulating six woody legume species (Acacia auriculiformis, Acacia senegal, A. seyal, Acacia tortilis ssp. raddiana, Leucaena leucocephala and Prosopis juliflora). Sequencing and phylogenetic analyses of nodA, nodC and nifH genes pointed out that in contrast to nodA gene, the phylogenies of nodC and nifH genes were not consistent with that of 16S rRNA, indicating that these genes of the A. seyal-nodulating rhizobia might have different origins. Microbial inoculation on nonsterile soil had significant effect on the nodules number and the growth of the seedlings, indicating that these strains of rhizobia might be used as inoculum. Conclusions: The results indicated that A. seyal is a nonselective host that can establish effective symbiosis with Mesorhizobium spp. strains from diverse genomic backgrounds and that the selected A. seyal-nodulating rhizobia could enhance plant growth. Significance and Impact of the Study: These results showed the important role that A. seyal could play in the improvement of reafforestation process as a promiscuous host, which can establish effective symbiosis with rhizobia from diverse genomic backgrounds.

[Departement_IRSTEA]Eaux [TR1_IRSTEA]BELCA; International audience; Acacia senegal (L) Willd. and Acacia seyal Del. are highly nitrogen-fixing and moderately salt tolerant species. In this study we focused on the genetic and genomic diversity of Acacia mesorhizobia symbionts from diverse origins in Senegal and investigated possible correlations between the genetic diversity of the strains, their soil of origin, and their tolerance to salinity. We first performed a multi-locus sequence analysis on five markers gene fragments on a collection of 47 mesorhizobia strains of A. senegal and A. seyal from 8 localities. Most of the strains (60%) clustered with the M. plurifarium type strain ORS 1032T, while the others form four new clades (MSP1 to MSP4). We sequenced and assembled seven draft genomes: four in the M. plurifarium clade (ORS3356, ORS3365, STM8773 and ORS1032T), one in MSP1 (STM8789), MSP2 (ORS3359) and MSP3 (ORS3324). The average nucleotide identities between these genomes together with the MLSA analysis reveal three new species of Mesorhizobium. A great variability of salt tolerance was found among the strains with a lack of correlation between the genetic diversity of mesorhizobia, their salt tolerance and the soils samples characteristics. A putative geographical pattern of A. senegal symbionts between the dryland north part and the center of Senegal was found, reflecting adaptations to specific local conditions such as the water regime. However, the presence of salt does not seem to be an important structuring factor of Mesorhizobium species.

Nitrate in the unsaturated zone between the soil surface and the water table was studied in agroforestry Parklands in north western Senegal by examination of samples obtained by hand auger. Depending on location, water tables existed at depths between 10 and 35m below ground. Previous studies of groundwater in this region had found that large concentrations of nitrate were unconnected with anthropogenic activity. The objective of this study was to determine whether nitrogen fixing vegetation had a role in groundwater nitrate accumulation and whether roots of trees were located deeply enough to access the nitrate. Accordingly, sample profiles were augered close to stems of nitrogen fixing trees, non-nitrogen fixing trees and also in adjacent areas that were unaffected by tree presence. These adjacent areas were typically open pasture or cultivated fields. Tree fine roots were quantified in the samples and examined for the presence of mycorrhizas. Similarly, sand/soil samples were examined and tested for the presence of nitrogen fixing rhizobia that were capable of forming functional nodules on appropriate host plants. Concentrations of nitrate were greatest in soils beneath nitrogen fixing trees and nitrate was more plentiful in profiles augered beneath nitrogen fixing crops than it was elsewhere suggesting that N-fixation was the source of the nitrate. The concentrrations of nitrate that were found in the unsaturated zone were greatly in excess of the WHO recommended limit for nitrate in drinking water. High NO3-N/Cl ratios confirm insitu production of nitrate, and indicate that this is a natural baseline occurrence related to N-fixation. The nitrate is moving down the profile and impacts the groundwater unless it can be intercepted by plant roots. NO3-N amounts in solution in the soil profile varied between 75 and 1000kg ha−1 beneath trees and between 120 and 400kg ha−1 in areas outwith tree crowns. Although these quantities of N occupy the lower end of the range of N values obtained in north American deserts, they comprise a considerable dryland resource where amounts of organic fertilizer are limited and where cost prohibits the use of commercial fertilizers. Roots of both nitrogen fixing and non-nitrogen fixing trees were deep enough to access the nitrate but the small amounts of available water at intermediate depths suggest that large scale uptake of nitrate will only be possible in the wetter zones located close to the water table. Shallow roots tended to be more heavily colonized by mycorrhizas than deeper roots but mycorrhizas were recovered from roots located 22m below ground. Tree roots and rhizobia had similar patterns of distribution. They were commonest close to the soil surface, less frequent at intermediate depths and tended to increase in frequency close to the water table.

Acacia holosericea seedlings were planted in 1-l pots filled with a soil collected from an Australian Acacia plantation in Southern Senegal. After 6 months of culture, mycorrhizosphere soil, roots, galls induced by root-knot nematodes and Rhizobium nodules were sampled from each pot. The diversity of this bacterial group was characterized by siderotyping (pyoverdine IsoElectric Focusing (IEF) analysis) and by restriction fragment length polymorphism (RFLP). The effect of these isolates on the establishment of the ectomycorrhizal symbiosis between an Australian Acacia (A. holosericea) and Pisolithus sp. strain IR100 was studied. In the mycorrhizosphere soil, the population of fluorescent pseudomonads was represented by strains of two different siderovars (groups of bacterial strains presenting an identical pyoverdine-IEF pattern): siderovar 1 (74%) and siderovar 2 (26%). The siderotyping of the isolates around galls of the root-knot nematodes revealed three siderovars (40% from siderovar 1, 40% from siderovar 2 and about 15% from siderovar 3). RFLP of 16S rDNA divided the isolates into four different groups with MspI, two with HhaI and two with HaeIII endonucleases. The establishment of the ectomycorrhizal symbiosis with A. holosericea was promoted by 14 bacterial strains isolated from the mycorrhizosphere soil, three isolates from the roots and four from the galls. Shoot biomass of A. holosericea seedlings was stimulated by eight bacterial isolates from soil, six isolates from galls and seven from roots. These mycorrhiza helper bacteria could have a great ecological importance in tropical areas through the reforestation programs.

Summary A total of fifty root nodules isolates of fast-growing and slow growing rhizobia from Pterocarpus erinaceus and Pterocarpus lucens respectively native of sudanean and sahelian regions of Senegal were characterized. These isolates were compared to representative strains of known rhizobial species. Twenty-two new isolates were slow growers and twenty-eight were fast growers. A polyphasic approach was performed including comparative total protein sodium dodecyl sulphate polyacrylamide gel (SDS-PAGE) profile analysis; 16S rDNA and 16S–23S rDNA intergenic spacer (IGS) sequence analysis. By SDS-PAGE the slow growing isolates grouped in one major cluster containing reference strains of Bradyrhizobium sp. including strains isolated in Africa, in Brazil and in New Zealand. Most of the fast-growing rhizobia grouped in four different clusters or were separate strains related to Rhizobium and Mesorhizobium strains. The 16S rDNA and 16S–23S rDNA IGS sequences analysis showed accurately the differentiation of fast growing rhizobia among the Rhizobium and Mesorhizobium genospecies. The representative strains of slow growing rhizobia were identified as closely related to Bradyrhizobium elkanii and Bradyrhizobium japonicum. Based on 16S rDNA sequence analysis, one slow growing strain (ORS199) was phylogenetically related to Bradyrhizobium sp. (Lupinus) and Blastobacter denitrificans. This position of ORS 199 was not confirmed by IGS sequence divergence. We found no clear relation between the diversity of strains, the host plants and the ecogeographical origins.

Nous avons étudié une collection de 76 nouveaux isolats bactériens de nodules obtenus par piégeage sur plantes d'Acacia tortilis subsp. raddiana in vitro, inoculées avec des suspensions de sols prélevés dans divers sites au nord (Tunisie) et au sud (Sénégal, Mauritanie) du Sahara, en comparaison avec 54 souches de références. L'analyse des nouveaux isolats par RAPD et SDS-PAGE a montré une grande diversité taxonomique parmi les isolats d'A. tortilis subsp. raddiana. En effet, bien que toutes les souches puissent être qualifiées de « rhizobium à croissance rapide », elles se répartissent en au moins 9 groupes différents, dont plusieurs ne correspondent à aucun des groupes de rhizobia déjà décrits. Aucune relation entre la position taxonomique des souches, leur origine géographique, les conditions édaphiques ou la profondeur d'isolement n'a pu être établie. La caractérisation symbiotique des souches d'A. tortilis subsp. raddiana a, au contraire, montré une grande homogénéité des caractéristiques symbiotiques majeures. Toutes les souches nodulent également A. Senegal, Prosopis julifora et Leucaena leucocephala, qui constituent avec A. tortilis subsp. raddiana un groupe d'inoculation. L'analyse des profils chromatographiques des facteurs Nod (FN) d'une quinzaine de souches réparties dans les différents groupes taxonomiques montre qu'ils sont très similaires. L'homogénéité structurale des FN des souches d'A. tortilis subsp. raddiana a été confirmée par la détermination de la structure des FN de quelques souches représentatives. Cette étroite relation entre FN et plante d'isolement/spectre d'hôte indépendamment de la taxonomie des souches suggère l'existence d'une forte pression de sélection de la part de la plante hôte et confirme nos observations précédentes, à savoir que les facteurs Nod sont de bons indicateurs du potentiel de nodulation d'un rhizobium donné. We studied a collection of 76 new bacterial isolates from nodules developed on Acacia tortilis subsp. raddiana plants inoculated in vitro with soil suspensions harvested in different sites in North (Tunisia) and South (Senegal, Mauritania) parts of Sahara. RAPD and SDS-PAGE analyses in comparison with 54 reference strains from diverse origins showed great taxonomie diversity among the new isolates. All of them are fast-growing and form at least 9 different groups, several of which could correspond to undescribed groups. No relationship could be established between taxonomie groups, geographical origin (North/South of Sahara) of the strains, edaphic conditions, or depth of soil sampling (down to - 32 m). Opposite to their taxonomie diversity, the new isolates share identical main symbiotic properties. All strains nodulating A. tortilis subsp. raddiana can also nodulate Prosopis juliflora, Acacia Senegal and Leucaena leucocephala, these four plants then constituting a cross-inoculation group. By thin-layer chromatography, we showed that 20 A. tortilis subsp. raddiana strains, representative of the different taxonomie groups, share very similar Nod factor (NF) profiles, independently from their geographical origins. NF structure homogeneity was confirmed by NF structure determination of some representative strains. The consistency between NF and plant of isolation/nodulation spectrum, independently of the taxonomie position of the strains suggests a high selection pressure from the plant, and confirms our previous observations that NF are good markers for predicting the nodulation potential of a given rhizobial strain.

Fifty-eight new isolates were obtained from root nodules of common bean (#Phaseolus vulgaris$) cultivated in soils originating from different agroecological areas in Senegal and Gambia (West Africa). A polyphasic approach including both phenotypic and genotypic techniques was used to study the diversity of the 58 #Rhizobium$ isolates and to determine their taxonomic relationships with reference strains. All the techniques performed, analysis of multilocus enzyme electrophoretic patterns, SDS-PAGE profiles of total cell proteins, PCR-RFLP analysis of the genes encoding 16S rRNA and of the 16S-23S RNA intergenic spacer region (ITS-PCR-RFLP), auxanographic tests using API galleries and nodulation tests lead to the consensus conclusion that the new rhizobial isolates formed two main distinct groups, I and II, belonging to #Rhizobium tropici$ type B and #Rhizobium etli$, respectively. By MLEE #R. etli$ and group II strains showed several related electrophoretic types, evidencing some extent of internal heterogeneity among them. This heterogeneity was confirmed by other techniques (ITS-PCR-RFLP, SDS-PAGE and host-plant-specificity) with the same nine distinct strains of group II showing some differences from the core of group II (54 strains). (Résumé d'auteur)

Thirty isolates of root-nodulating bacteria obtained from Acacia cyanophylla, A. karroo, A. cyclops, A. tortilis (subsp.raddiana), Faidherbia albida and Acacia sp., grown in different regions of Libya, were studied by performing numerical analysis of 104 characteristics. Three fast- and one slow-growing reference strains from herbaceous and woody legumes were included. Five distinct clusters were formed. The fast-growing reference strains were separated from the isolates whereas the slow-growing was included in cluster 4. With the exception of one cluster, the majority of clusters were formed regardless of the host plant or site of origin. Based on plant tests, generation times, acid production and carbon utilization the isolates were diverse (fast and slow-growing isolates). Like slow-growing isolates, most of the fast-growing isolates appeared to be non-specific, nodulated many species from the same genus notably F. albida, known to nodulate only with slow-growing strains. Most clusters grew at temperatures 35 °C and 37 °C; some grew at temperatures above 40 °C. The majority of isolates grew at acid and alkaline pH and only one isolate grew below pH 4. Most isolates were able to utilize many amino acids as nitrogen sources and to reduce nitrate. Urea was hydrolysed by all clusters. Monosaccharides and polyols were used by slow and fast-growing isolates as the only carbon sources whereas assimilation of disaccharides varied: Some isolates, like slow-growing isolates, failed to utilize these carbon sources. Most isolates were unable to utilize polysaccharides. Regarding tolerance to NaCl on agar medium, the majority of isolates were unable to grow at a concentration of 2% NaCl, but some were highly resistant and there was one isolate which grew at 8% NaCl. Most isolates were resistant to heavy metals and to antibiotics.

The genetic diversity of native cowpea rhizobia originating from 60 sites across four eco-geographic zones in Senegal Was studied. More than 300 cowpea nodules were analyzed by PCR-RFLP of the 16S-23S rDNA InterGenic Spacer region (IGS). Alignments of IGS sequences indicated that all genotypes were grouping within the Bradyrhizobium genus. The geographical distribution showed that apart from five IGS types, the others were specifically found in only one region. The diversity was significantly higher in the Senegal River valley zone, which presents lower mean annual rainfalls and slightly alkaline soils. Interestingly, two IGS types dominated the Senegalese rhizobial collection, one IGS type (VI) was found on more than half of the nodules collected in the northern Senegal River valley while another IGS type (I) was recovered from the great majority of nodules in the three other regions sampled. Two representative strains from each of these two dominant types were isolated and further analyzed. Multi Locus Sequence Analyses using 6 housekeeping genes indicate that they belong to a new Bradyrhizobium species closely related to B. yuanmingense. Phylogenetic analyses of 2 symbiotic genes nodC and nifH show that they are clustered with B. arachidis. Physiological tests on these strains have shown that under laboratory conditions, the growth of the IGS type VI strains was slightly less affected by a higher osmotic strength in the medium and to alkaline pH, which corroborates the soil physico-chemical parameters. (C) 2013 Elsevier GmbH. All rights reserved.

La culture du Niebe [Vigna unguiculata (L.) Walp.] en Afrique de l’Ouest est marquee par l’instabilite de ses rendements et sa faible production liee a la pauvrete des sols, a la faiblesse de la pluviometrie et aux contraintes parasitaires. Dans cette etude, nous nous sommes interesses a la reponse a l’inoculation mycorhizienne de quatre varietes de niebe dans une perspective de selection de souches efficaces de champignons endomycorhiziens pour l’amelioration de sa productivite au champ. Quatre varietes de niebe dont deux du Burkina Faso [Gorom Local (GO) et KVX 396-4-5-2D (KVX)] et deux du Senegal [Melakh (Mel) et Yacine (Yac)] ont ete inoculees en pepiniere avec trois (3) inoculums mixtes du Burkina Faso [Yakouta (Ya), Tale Mossi (Ta) et Soumousso (Sou)] et trois inoculums monospecifiques de la collection du Laboratoire Commun de Microbiologie de Dakar (LCM) [Glomus mossea (Gm), Glomus fasciculatum (Gf) et Glomus intraradices (Gi)]. Les effets des differents inoculums sur la croissance des plants ont ete evalues par la mesure de la hauteur des plants, des biomasses aerienne et racinaire et de la frequence et intensite de mycorhization a la floraison et a la fructification. Les resultats obtenus montrent que l’inoculation n’a pas eu d’effets significatifs sur les differents parametres mesures pour les varietes du Senegal malgre des taux de mycorhization de 6% a 92% a la floraison et de 44% a 100% a la fructification. Pour les varietes du Burkina Faso, a la floraison, la croissance en hauteur et la production de biomasse aerienne de la variete KVX ont ete ameliorees par toutes les souches, exceptee Gm. Pour la variete GO, les inoculums mixtes indigenes Ya et Ta ont significativement ameliore la croissance en hauteur des plants. En general, les frequences et les intensites de mycorhization sont elevees pour les souches monospecifiques, mais ceci ne se traduit pas par un effet significatif sur la croissance des plants inocules. L’inoculum mixte indigene Ya apparait plus performant pour la croissance en hauteur et la production de biomasse de la variete la plus sensible a l’inoculation (KVX). Cette etude a montre que l’inoculation endomycorhizienne a un effet benefique sur la croissance du Niebe, particulierement la variete KVX ; et que l’utilisation de souches indigenes conviendrait mieux que les souches monospecifiques. Il s’avere donc interessant de poursuivre la selection de souches performantes pour la productivite du Niebe sur les souches indigenes. Mots cles : champignons mycorhiziens, croissance du Niebe, pepiniere, Afrique de l’Ouest

International audience; Increasing interest has been given so far to the role of mycorrhizal symbiosis on plant diversity and ecosystem productivity. However much remains unknown about the effect of ectomycorrhizal fungal diversity on plant growth and rhizobial symbiosis. The purpose of this study is to investigate the influence of ectomycorrhizal diversity on root nodulation and plant nutrient uptake during plant growth. A gradient of ectomycorrhizal diversity was created (ranged from one to six fungal isolates) on Australian Acacia (Acacia mangium) seedlings grown on a phosphate-deficient soil in controlled conditions. The results dearly show (i) an enhancement of A. mangium growth due to ectomycorrhizal fungal inoculation, (ii) as well as significant effects of increasing ectomycorhizal diversity and soil ectomycorrhizal mycelium density on whole plant biomass and plant nutrient uptake. They also reveal an effect of ectomycorrhizal diversity effect on the functioning of the nitrogen-fixing symbiosis. These results emphasize the need to manage soil ectomycorrhizal potential (abundance and diversity of ectomycorrhizal mycelium) in forest management practices in order to optimize N-2 fixation and seedling growth, leading thus to dramatic improvements in outplanting performances with fast-growing tree legumes on various reafforestation sites, more particularly in tropical and Mediterranean areas. (C) 2012 Elsevier Ltd. All rights reserved.

Assessment of diversity and understanding factors underlying species distribution are fundamental themes in ecology. However, the diversity of native arbuscular mycorrhizal fungi (AMF) species in African tropical agro-ecosystems remains weakly known. This research was carried out to assess the morphological diversity of indigenous AMF species associated with rhizosphere of cowpea (Vigna unguiculata (L.) Walp.) in different agro-ecological zones (AEZ) of Benin and to examine the effects of soil chemical properties, climatic factors and agricultural practices on this diversity. Results showed that, in Benin, cowpea was grown by farmers in very exhausted soils, where available phosphorus and potassium were deficient. The indigenous AMF spore density was on average 202 spores per 100 g dry soil and there was no difference in the density among the agro-ecological zones (P = 0.56). Fifteen AMF morphospecies belonging to eight genera (Gigaspora, Scutellospora, Racocetra, Acaulospora, Funneliformis, Rhizophagus, Glomus and Claroideoglomus) were detected. The computed species richness estimators indicated that a limited number of additional undetected morphospecies are probably present in cowpea fields. According to analysis of similarity (ANOSIM), the arbuscular mycorrhizas community composition of the Sudanian zone and Guinean zone were not significantly different (r = −0.01; P = 0.517). The diversity of AMF morphospecies in the fields was weak with prevalence of Glomeraceae (92%). Furthermore, AMF diversity and evenness indices were negatively correlated with annual rainfall (P < 0.01) and with available phosphorus (P < 0.05). However, no significant correlation was observed between AMF diversity indices and soil organic carbon. Ultimately, this study tends to confirm that soil management practices (tillage, soil disturbance) have negative effect on AMF diversity.

Streams draining agricultural basins are subjected to the input of fungicides which can affect aquatic microbial communities. We analyzed the effect of the fungicide tebuconazole (TBZ) on Alnus glutinosa and Populus nigra litter breakdown by aquatic microorganisms. For six weeks, fungal and bacterial responses were analyzed in indoor stream channels subjected to TBZ-contaminated (33.1±12.4 μg L(-1)) and uncontaminated conditions. Litter breakdown rates decreased in presence of TBZ. The decrease was explained by reductions in microbial biomass development and shifts in community structure. At the same time, TBZ modified the kinetics of β-glucosidase, β-xylosidase and cellobiohydrolase enzymes resulting in lower affinities for cellulose and hemicellulose decomposition in leaves. These alterations were modulated by the litter quality; the greatest structural impairment was observed in Populus whereas Alnus were more affected in terms of leaf breakdown rate. Our results suggest that chronic exposure to TBZ can affect aquatic microbial communities and their capacity to break down leaf litter in streams.

Présentation du contexte Les systèmes écologiques des zones africaines sont soumis depuis des décennies à d’importantes perturbations climatiques (effets de la sécheresse et des fortes températures) et anthropiques (surexploitation des terres, modification des pratiques culturales). Ainsi, on assiste généralement à un accroissement de l’aridité d’origine édaphique et à une modification du couvert végétal et des paysages, qui affectent la productivité des systèmes de production et les conditio...

Many cowpea varieties (Vigna unguiculataL. Walp) have been proposed by research in Mali. None of these varieties were investigated for their symbiotic potential in terms of root nodulation and mycorrhizal infection. An experiment was conducted at Cinzana Research Station, from 2007 to 2009 with an objective to identify a cowpea variety with high symbiotic potential which may improve millet/cowpea cropping global production. Randomized complete block (RCBD) design with a factorial combination of 3 cowpea varieties (IT89KD-374, CZ1-94-23-1, and CZ11-94-5C) and 2 cropping systems (millet/cowpea intercropping and cowpea-millet rotation) was used. On farm test was conducted to evaluate CZ11-94-5C and IT89KD-374 nodulation performance. Cowpea variety CZ11-94-5-C had the highest nodule number and nodule weight. Millet/cowpea alternate rows intercropping (1/1), only, had a significant influence on cowpea root infection rates by mycorrhizae, on the 45th day after emergence. IT89KD-374 gave the best cowpea grain yield (1540 kg ha−1) in sole crop. The highest millet grain yield (1650 kg ha−1) was obtained under CZ11-94-5C-millet rotation. Farmers' fields assessments results confirmed CZ11-94-5C performance on research station. The CZ11-94-5C cowpea variety needs to be more characterized.

Il est désormais fréquent que des chercheurs en sciences sociales soient sollicités dans des programmes de recherche sur l'environnement et la biodiversité. Ce type de collaboration mobilise une attention croissante à l'hétérogénéité des formes de production et de légitimation des savoirs, d'autant plus forte lorsqu'il s'agit de programmes qui impliquent des experts non-scientifiques. Ce contexte génère des tensions, entre d'une part, la rationalisation gestionnaire des formes du travail scientifique et notamment de son évaluation standardisée, et d'autre part, l'ouverture épistémologique à la diversité des modes de légitimation des savoirs et par conséquent à une créativité dans les formes du travail qui contrecarre les tendances à la standardisation. C'est pour rendre compte de l'attention à la pluralité des savoirs et de ce qu'elle implique et qu'elle crée précisément dans le courant d'une recherche collective, que nous revenons sur une démarche photo-ethnographique menée dans le cadre d'un programme de recherche participatif et interdisciplinaire au Sénégal. Cette démarche a permis de : (i) recueillir des données sur les savoirs locaux et les savoirs scientifiques ; (ii) participer au partage des savoirs propres aux différents acteurs impliqués et (iii) restituer et diffuser des résultats de la recherche partenariale. Mais elle a surtout constitué une forme d'expérimentation collective de formes de communication, de mise en discussion et en circulation des savoirs singuliers mobilisés et produits dans le cadre du projet. Au travers de cette démarche photo-ethnographique est révélée l'importance de la construction d'une sémiotique pratique commune à l'ensemble des acteurs pour le partage effectif de savoirs multiples dans le cadre de recherche collaborative. Cette étude souligne la manière dont toute construction et toute expression d'un savoir quel qu'il soit est liée à des situations de communication. Une démarche réflexive sur les modes de co-construction de connaissance implique donc nécessairement un travail d'analyse et d'expérimentation sur les pratiques de communication dans des recherches qui associent des collectifs différents.

A total of 80 bacterial strains isolated from different Sesbania and Acacia species growing in various sites in Senegal (West Africa) were compared with 35 reference strains of Rhizobium, Bradyrhizobium, Azorhizobium, and Agrobacterium species and with 33 representative strains of the different groups of Brazilian isolates described on the basis of the results of a numerical analysis of the whole-cell protein patterns obtained by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Fifty-two strains could be placed in three protein electrophoretic clusters, two of which were different from the clusters containing various reference or representative strains, while 30 other strains could not be placed in any group. The strains belonging to the three clusters were studied by determining their nodulation host ranges and their morphological, physiological, and auxanographic characteristics. Representative strains of the three clusters were also genotypically characterized by determining their DNA base compositions, by performing DNA-DNA and DNA-rRNA hybridization experiments, and by determining their 16S rRNA gene sequences. Our results showed that two of the clusters identified on the basis of SDS-PAGE data are genotypically and phenotypically distinct groups that belong on the Rhizobium meliloti-Rhizobium fredii rRNA branch. The third cluster is localized on the Rhizobium loti rRNA branch in the vicinity of Rhizobium huakuii and contains strains isolated in Africa, in Brazil, and in New Zealand from different leguminous species. On the basis of the results of the present study, we propose to emend the genus Sinorhizobium and to reclassify R. meliloti as Sinorhizobium meliloti comb. nov. In addition, two new species, Sinorhizobium saheli and Sinorhizobium teranga, are proposed for isolates from Senegal.

Many fast growing tree species have been introduced to promote biodiversity rehabilitation on degraded tropical lands. Although it has been shown that plant productivity and stability are dependent on the composition and functionalities of soil microbial communities, more particularly on the abundance and diversity of soil symbiotic micro-organisms (mycorrhizal fungi and rhizobia), the impact of tree introduction on soil microbiota has been scarcely studied. This research has been carried in a field plantation of Acacia holosericea (Australian Acacia species) inoculated or not with an ectomycorrhizal fungus isolate, Pisolithus albus IR100. After 7 year's plantation, the diversity and the symbiotic properties of Bradyrhizobia isolated from the plantation soil or from the surrounding area ( Faidherbia albida (Del.) a. Chev. parkland) and able to nodulate F. albida , a native Sahelian Acacia species, have been studied. Results clearly showed that A. holosericea modified the structure of Bradyrhizobia populations and their effectiveness on F. albida growth. This negative effect was counterbalanced by the introduction of an ectomycorrhizal fungus, P. albus , on A. holosericea root systems. In conclusion, this study shows that exotic plant species can drastically affect genotypic and symbiotic effectiveness of native Bradyrhizobia populations that could limit the natural regeneration of endemic plant species such as F. albida . This effect could be counterbalanced by controlled ectomycorrhization with P. albus . These results have to be considered when exotic tree species are used in afforestation programs that target preservation of native plants and soil ecosystem rehabilitation.

Aims: We characterized phenotypically and genotypically root-nodulating bacteria associated with Acacia senegal (L.) Willd. isolated from the soils surrounding A. senegal trees in the dry land area of Senegal. Methods and Results: The phenotypical and genotypical characterizations we carried out showed a high diversity of A. senegal root-nodulating bacteria. Phenotypic patterns showed adaptations of the rhizobial strains to many environmental stresses such as heat, drought, and salinity. Twelve molecular groups were distinguished by profiles obtained using polymerase chain reaction/restriction fragment length polymorphism techniques from intergenic spacer region rDNA. The highest genetic diversity was found around the A. senegal rhizosphere. Therefore, A. senegal seemed to have a positive influence on occurrence and genotypical diversity of rhizobial populations. Rhizobial isolates obtained in this study belonged phylogenetically to the genera Mesorhizobium and Rhizobium. Conclusions: Our results provided information about the genetic diversity of the rhizobial strains associated with A. senegal and suggested the adaptability of natural rhizobial populations to major ecological environmental stress within these soil environments. Significance and Impact of the Study: These results suggested a potential selection of compatible and well adapted strains under stress conditions as inoculants for successful A. senegal growth in arid lands.

The occurrence and the distribution of rhizobial populations naturally associated to Acacia seyal Del. were characterized in 42 soils from Senegal. The diversity of rhizobial genotypes, as characterized by polymerase chain reaction restriction fragment length polymorphism (RFLP) analysis of 16S-23S rDNA, performed on DNA extracted from 138 nodules resulted in 15 clusters. Results indicated the widespread occurrence of compatible rhizobia associated to A. seyal in various ecogeographic areas. However, the clustering of rhizobial populations based on intergenic spacer (IGS) RFLP profiles did not reflect their geographic origin. Four genera were discriminated on the basis of 16S rRNA gene sequences of the strains representative for the IGS-RFLP profiles. The majority of rhizobia associated to A. seyal were affiliated to Mesorhizobium and Sinorhizobium 64 and 29%, respectively, of the different IGS-RFLP profiles. Our results demonstrate the coexistence inside the nodule of plant-pathogenic non-N(2)-fixing Agrobacterium and Burkholderia strains, which induced the formation of ineffective nodules, with symbiotic rhizobia. Nodulation was recorded in saline soils and/or at low pH values or in alkaline soils, suggesting adaptability of natural rhizobial populations to major ecological environmental stress and their ability to establish symbiotic associations within these soil environments. These results contribute to the progressing research efforts to uncover the biodiversity of rhizobia and to improve nitrogen fixation in agroforestry systems in sub-Saharan Africa.

Polymerase Chain Reaction/Restriction Fragment Length Polymorphism (PCR/RFLP) of the InterGenic Spacer (IGS) between rDNA 16S and 23S was used to identify indigenous strains nodulating four clones of Acacia mangium–Acacia auriculiformis hybrids cultivated in non-sterilized sandy soil from Sangalkam (Senegal) under greenhouse conditions. The experiment was for 4 months. The analysis of restriction fragment length polymorphism obtained with MspI and HaeIII restriction enzymes allowed the identification of 15 different IGS Groups with a distribution which significantly differed according to the clone of the hybrid (strains of one clone can belong to three and five different IGS Groups). Three large multi-lobed nodules were obtained on the root system of clone 3.26 within 5 months. Also, the nature of the rhizobia contained in each lobe was determined. The results showed that the lobes of large nodules can be occupied by one or two strains and the nodules analysed were mainly occupied by those belonging to IGS Group 12.

Aims: In order to understand the genetic diversity of Acacia tortilis ssp. raddiana-rhizobia in Tunisia, isolates from nine geographical locations were obtained and analysed. Methods and Results: Characterization using restriction fragment length polymorphism analysis (RFLP) of PCR-amplified 16S rRNA gene and the intergenic spacer (IGS) between the 16S and 23S rRNA genes was undertaken. Symbiotic efficiency of the strains was also estimated. Analysis of the 16S rRNA by PCR-RFLP showed that the isolates were phylogenetically related to Ensifer ssp., Rhizobium tropicii-IIA, and Rhizobium tumefaciens species. Analysis of 16S-23S spacer by PCR-RFLP showed a high diversity of these rhizobia and revealed eleven additional groups, which indicates that these strains are genetically very diverse. Full 16S rRNA gene-sequencing showed that the majority of strains form a new subdivion inside the genera Ensifer, with Ensifer meliloti being its nearest neighbour. Nodulation test performed on the plant host demonstrated differences in the infectivity among the strains. Conclusion: Rhizobial populations that nodulate specifically and efficiently Acacia tortilis ssp. raddiana in representative soils of Tunisia is dominated by E. meliloti-like genomospecies. Significance and Impact of the Study: This paper provides the first clear characterization and symbiotic efficiency data of rhizobia strains nodulating A. tortilis in Tunisia.

L'association d'Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan avec les bactéries du sol à croissance rapide de la famille des rhizobiacées joue un rôle important dans l'amélioration de la nutrition minérale de cette plante. Cependant, les caractéristiques de cette symbiose ont été très peu étudiées en conditions naturelles. Ce travail a été conduit au Sénégal sur des peuplements naturels d'Acacia raddiana de deux sites du Ferlo sénégalais : Souilène et Dahra-Djoloff. La répartition dans l'espace et les variations saisonnières des nodules présents sur le système racinaire de surface d'Acacia raddiana (0-1 m) ont été étudiées en relation avec les conditions hydriques du sol. À cet effet, des prélèvements de sol et des extractions ont été effectués à différentes périodes de l'année. Le maximum de nodules a été rencontré entre 2 et 3 mètres du tronc, et à une profondeur comprise entre 25 et 75 cm. Une biomasse importante de nodules apparemment très fixateurs d'azote a été produite pendant la saison pluvieuse. Ces nodules ont une durée de vie courte, et apparaissent desséchés pendant la majeure partie de l'année. La densité des nodules apparaît corrélée à la biomasse racinaire, mais également à l'humidité du sol de la rhizosphère. The association between Acacia tortilis (Forssk.) Hayne subsp. raddiana (Savi) Brenan and soil bacteria with rapid growth belonging to the Family Rhizobiaceae plays an important role in the improvement of the mineral nutrition of this plant. However, the characteristics of this symbiotic relationship have been very little studied under natural conditions. This work has been conducted in Senegalese natural populations of Acacia raddiana in two sites: Souilène and Dahra-Djoloff (Ferlo). The spatial distribution around tree and the seasonal variations of nodules present on the root system of Acacia raddiana (0-1 m) were studied under different soil moisture conditions. For that, extraction of soil samples was made at the different periods of the year. The maximum number of nodules was obtained between 2 and 3 m far from the tree trunk, and between 25 and 75 cm soil depth. A significant nodule biomass apparently more efficient in N fixation was obtained during the rainy season. These nodules have short life span, and appeared dried during the greater part of the year. The density of the nodules correlated significantly with the root biomass, and also with the soil moisture content of the rhizosphere. nodulation, root, natural population, soil water budget, acacia tortilis (forssk.), hayne subsp. raddiana (savi), brenan

The diversity of Bradyrhizobium strains nodulating three cowpea (Vigna unguiculata L. Walp.) cultivars in favorable and water-limited conditions occuring at flowering was analysed. PCR- Restriction Fragment Length Polymorphism (PCR-RFLP) analysis of 16S-23S rDNA intergenic spacer region (IGS) directly applied on 85 crushed nodules distinguished four genetic profiles, IGS types I, II, III and IV. The distribution of these IGS types according to water conditions and cowpea cultivars (B-21, TN 88-63 and Mouride) showed that nodulating strains appeared more diverse in water-limited condition. More than three quarters of prospected nodules presented the IGS type I. They were formed on all three cultivars and in both water conditions. Only a small part of nodules was distributed between the IGS type II, III and IV. Nodules showing the IGS types II and III were found mainly in limited conditions on TN 88-63 and Mouride cultivars, whereas nodules presenting the IGS type IV were collected only from cultivars B-21 and Mouride, in both water conditions. Strains corresponding to the different profiles were isolated. The phylogenetic analysis of 16S rRNA gene sequences showed that they belong to the genus, Bradyrhizobium. The sequence analysis of 16S-23S rDNA IGS revealed that the strains exhibiting IGS types II, III and IV were closely related to some Faidherbia albida isolates from Senegal. IGS type II can be assigned with at least 98% similarity to Bradyrhizobium genospecies IV. IGS types III and IV showed more than 96% similarity with genospecies VII and could belong to the same genospecies. IGS type I, the most frequent, exhibits low IGS similarity with reported sequences in the databases, and could represent a new genospecies. (African Journal of Biotechnology: 2003 2(1): 13-23)

International audience; Our work concerned the optimization of inoculation of two agroforestry species of woody leguminous plants: Leucaena leucocephala and Acacia mangium with various strains of rhizobium. First, we showed that the physiological stage of the bacterial culture had no effect on nodulation and growth of the seedlings of Acacia mangium inoculated and cultivated in vitro for four months. For Leucaena leucocephala, the number of nodules was significantly higher when the seedlings were inoculated with a bacterial culture in stationary phase. On the other hand, whatever the species, no significant difference was noted with regards to the dry weight of the shoots. The effect of the size of inoculum on the nodulation and growth of the seedlings was studied in L. leucocephala after five months in a greenhouse. Our result show that an inoculation with bacterial cultures containing 10$^9$ to 10$^{10}$ bacteria per milliliter are the optimal conditions to have a maximum nodulation and growth of the seedlings. The two legume plant species showed significant differences with regard to the effect of the method of inoculation on nodulation and growth of the seedlings. For Acacia mangium, inoculation with a liquid culture one week after sowing was more favourable for the growth of the seedlings. On the other hand, for Leucaena leucocephala, this method of inoculation and the coating of seeds with a bacterial culture mixed with arabic gum improved significantly the growth of the seedlings. Results obtained in our study can be useful for the partners from developing countries involved in the large scale production of tree seedlings.; Optimisation de l'inoculation de Leucaena leucocephala et de Acacia mangium avec rhizobium en conditions de serre. Nos travaux ont porté sur l'optimisation de l'inoculation de 2 espèces agroforestières de légumineuses ligneuses : Leucaena leucocephala et Acacia mangium avec différentes souches de rhizobium. Tout d'abord, nous avons montré que le stade physiologique de la culture bactérienne n'a aucune influence sur la nodulation et la croissance des plants d'Acacia mangium inoculés et cultivés in vitro pendant 4 mois. Pour Leucaena leucocephala, le nombre de nodules est significativement supérieur pour les plants inoculés avec une culture bactérienne en phase plateau. En revanche, quelle que soit l'espèce, aucune différence significative n'est notée en ce qui concerne le poids sec des parties aériennes. L'effet de la taille de l'inoculum sur la nodulation et la croissance des plants de L. leucocephala a été étudié après 5 mois de culture en serre. Nos résultats ont montré qu'une inoculation avec des cutures bactériennes contenant 10$^{10}$ à 10$^9$ bactéries par millilitre sont les conditions optimales pour avoir une nodulation et une croissance maximale des plants. Les 2 espèces de légumineuses montrent des différences significatives en ce qui concerne l'effet de la méthode d'inoculation sur la nodulation et la croissance des plants. Pour Acacia mangium, l'inoculation avec une culture liquide une semaine après les semis est plus favorable pour la croissance de la plante. En revanche, pour Leucaena leucocephala, cette méthode classique d'inoculation de même que l'enrobage des semis avec une culture bactérienne mélangée avec de la gomme arabique améliorent significativement la croissance de la plante. Nos travaux pourront servir aux partenaires du développement pour la production en régie de plants forestiers de ces 2 espèces.

The aim of the study was to assess the effect of the mycorrhizosphere of A. tortillis spp. raddiana mycorrhized with Glomus intraradices on the root nodulation by Sinorhizobium terangae (ORS 1009) and/or Mesorhizobium plurifarium (ORS 1096) in two different culture substrates (sandy soil and sand). The endomycorrhizal fungus only stimulated plant growth in the sandy soil. Moreover, arbuscular mycorrhizal infection enhanced the nodulation process in both culture substrates. Beside the stimulatory effects of the mycorrhizosphere on both rhizobia development, fungal symbiosis induces two different dynamics of each bacterial strains in the sand-grown plants. These results suggest specific relationships could occur during the development of the tripartite symbiosis, at physiological and molecular level. From a practical point of view, the role of arbuscular mycorrhizas in improving nodulation and N2 fixation is universally recognized. The fungal symbiosis could modify the development of bacterial inoculants along the root systems. This effect is of particular interest in the controlled inoculation of selected rhizobia.

A collection of rhizobia isolated from Acacia tortilis subsp. raddiana from various sites in the North and South of Sahara was analyzed for their diversity at both taxonomic and symbiotic levels. On the basis of whole cell protein (SDS-PAGE) and 16S rDNA sequence analysis, most of the strains were found to belong to the Sinorhizobium and Mesorhizobium genera where they may represent several different genospecies. Despite their chromosomal diversity, most A. tortilis Mesorhizobium and Sinorhizobium symbionts exhibited very similar symbiotic characters. Nodulation tests showed that the strains belong to the Acacia-Leucaena-Prosopis nodulation group, although mainly forming non-fixing nodules on species other than A. tortilis. Most of the strains tested responded similarly to flavonoid nod gene inducers, as estimated by using heterologous nodA-lacZ fusions. Thin layer chromatography analysis of the Nod factors synthesized by overproducing strains showed that most of the strains exhibited similar profiles. The structures of Nod factors produced by four different Sinorhizobium sp. strains were determined and found to be similar to other Acacia-Prosopis-Leucaena nodulating rhizobia of the Sinorhizobium-Mesorhizobium-Rhizobium branch. They are chitopentamers, N-methylated and N-acylated by common fatty acids at the terminal non reducing sugar. The molecules can also be 6-O sulfated at the reducing end and carbamoylated at the non reducing end. The phylogenetic analysis of available NodA sequences, including new sequences from A. tortilis strains, confirmed the clustering of the NodA sequences of members of the Acacia-Prosopis-Leucaena nodulation group.

Summary • The influence of two fluorescent pseudomonads strains (HR13 and HR26) on the ectomycorrhizal symbiosis between Pisolithus alba and Acacia holosericea is reported here. • We measured ectomycorrhizal establishment, fungal growth in the soil (by HPLC) and soil microbial biomass (using the fumigation–extraction method) in treatments with or without pseudomonads. • Bacteria inoculated with the fungal symbiont stimulated ectomycorrhizal formation and shoot or root biomass. Only HR13 significantly increased fungal biomass in the soil. The bacteria stimulated fungal growth and production of phenolic compounds. Sequence analysis of the two fluorescent Pseudomonas revealed 99% homologuey between HR13 and P. monteilii, and 98% between HR26 and P. resinovorans. • It is clear that some bacteria (Mycorrhiza Helper Bacteria) can stimulate the establishment of the ectomycorrhizal symbiosis in tropical conditions.

Rhizobia described so far belong to three distinct phylogenetic branches within the α-2 subclass of Proteobacteria . Here we report the discovery of a fourth rhizobial branch involving bacteria of the Methylobacterium genus. Rhizobia isolated from Crotalaria legumes were assigned to a new species, “ Methylobacterium nodulans ,” within the Methylobacterium genus on the basis of 16S ribosomal DNA analyses. We demonstrated that these rhizobia facultatively grow on methanol, which is a characteristic of Methylobacterium spp. but a unique feature among rhizobia. Genes encoding two key enzymes of methylotrophy and nodulation, the mxaF gene, encoding the α subunit of the methanol dehydrogenase, and the nodA gene, encoding an acyltransferase involved in Nod factor biosynthesis, were sequenced for the type strain, ORS2060. Plant tests and nodA amplification assays showed that “ M. nodulans ” is the only nodulating Methylobacterium sp. identified so far. Phylogenetic sequence analysis showed that “ M. nodulans ” NodA is closely related to Bradyrhizobium NodA, suggesting that this gene was acquired by horizontal gene transfer.

Six provenances of #Acacia auriculiformis$ have been tested for their ectomycorrhizal dependency with 2 indigenous strains of #Pisolithus$ sp. Moreover, the endomycorrhizal and rhizobial symbiosis, the multiplication of plant parasitic nematodes have been quantified. The results showed significant variations in plant development between provenances. Both the compatibility with and dependency on the ectomycorrhizal strains were different among the provenances. Three main genera of nematodes have been identified, in particular #S. cavenessi$, which is one of the most pathogenic nematodes of soudano-sahelian crops. Endomycorrhizal structures were sometimes detected on the root systems. These results suggest that provenances of #A. auriculiformis$ that are well adapted to the indigenous environmental conditions (pedoclimatic, soil borne pathogens) must be selected for their mycorrhizal dependency and for their resistance to plant parasitic nematodes. Moreover, the ectomycorrhizal symbiosis has to be considered as an important component of the cultural system. (Résumé d'auteur)

A high-resolution phylogenetic analysis of Nitrobacter strains and their neighbours was made using the rrs-rrl intergenic spacer sequence and the hypervariable part of the rrl gene. The phylogenetic tree obtained was consistent with that which was obtained previously but was much more discriminating, permitting the design of genus-specific primers.

International audience; The ectomycorrhization of Acacia holosericea with fungi isolated in Senegal (belonging to the Pisolithus genus) and their effect on Meloidogyne javanica have been studied. In 100 tree plantations of Eucalyptus camaldulensis, Casuarina equisetifolia, Acacia mangium and A. holosericea, 33 fruiting bodies of the ectomycorrhizal fungus Pisolithus were collected and cultured under axenic conditions. Only four fungal isolates have induced the formation of typical ectomycorrhizae with A. holosericea under axenic conditions. One of these, COI 024, increased plant development under glasshouse conditions and decreased the multiplication of the root-knot nematode Meloidogyne javanica. The mechanisms involved in these interactions and the potential use of the ectomycorrhizal symbiosis in agroforestry programs are being discussed.; Ectomycorhization de Acacia holosericea A. Cunn. ex G. Don par Pisolithus spp. au Sénégal : effet sur le développement de la plante et sur la multiplication du nématode à galles Meloidogyne javanica. L'effet de symbiotes fongiques ectomycorhiziens, appartenant au genre Pisolithus et isolés au Sénégal, a été étudié sur la croissance de plants d'Acacia holosericea et sur le développement d'un peuplement de nématodes phytoparasites Meloidogyne javanica. Une enquête a été réalisée sur 100 arbres choisis dans différentes plantations de Eucalyptus camaldulensis, Casuarina equisetifolia, Acacia mangium et A. holosericea où trente trois carpophores de Pisolithus spp. ont été prélevés. Le mycélium issu de chaque carpophore a été cultivé en conditions axéniques et chaque souche a été testée pour sa compatibilité avec des plants d'A. holosericea. Seulement 4 isolats ont induit la formation d'ectomycorhizes en conditions axéniques. Dans une expérience conduite en serre avec A. holosericea, une souche, COI 024, a stimulé la croissance de la plante hôte et inhibé la multiplication de Meloidogyne javanica. Les mécanismes susceptibles d'être à l'origine de ces phénomènes sont discutés et les effets bénéfiques potentiels du recours à la mycorhization contrôlée dans les programmes agroforestiers sont exposés.

L'analyse du polymorphisme de longueur des fragments de restriction d'ADN amplifie (PCR-RFLP) presente de nombreux avantages pour la caracterisation de la diversite microbienne: relative rapidite, faible cout, possibilite d'application a l'ensemlble ries taxons bacteriens, bonne reproductibilite, possibilite d'analyse simultanee d'un grand nombre de souches, interet au niveau taxonomique. Afin d'optimiser sort usage, nous avons cherche a determiner combien d'enzymes et lesquels etaient le plus approprie, en effectuant une analyse systematique par ordinateur de la capacite de 25 enzymes de restriction differentes a generer par PCR-RFLP, seules ou en combinaison, (les profils de digestion permettant la distinction d'especes proches taxonomiquement. La sous-unite 16S de l'ARN ribosomal (rrs; pour ribosomal RNA small subunit) a ete choisie pour simuler ces PCR-RFLP: elle correspond a la region la plus frequemment etudiee et permet en general une bonne discrimination au niveau de l'espece; de plus, de nombreuses sequences rrs sont disponibles dans les banques de donnees. L'etude a ete focalisee sur les rhizobia qui representent un bon modele de genres differents, entremeles avec d'autres genres tres proches taxonomiquement, et parce que leur importance en agriculture induit la necessite d'outils performants pour la caracterisation de grands nombres de nouveaux isolats. La digestion de 24 sequences rrs correspondant aux souches-types de differentes especes a ete simulee par ordinateur. Les profils de digestion obtenus pour les differentes sequences ont ete compares deux par deux afin de determiner le nombre de profils distincts generes par chaque enzyme individuellement. ou par les combinaisons systematiques de 2, 3, 4, 5, 6 ou 7 enzymes. Differentes combinaisons de 3, 4 ou 5 enzymes permettent la distinction entre les especes. La frequence a laquelle les differentes enzymes sont trouvees dans les i:ounfiinaisons discrimimantes ne correspond pas completement a leur pouvoir de discrimation individuel, mais a la complementarite qu'elles presentent lorsqu'ellas sont analysees ensemble. La combination des profils obtenus avec six enzymes, choisies parmi les sept puis frequentes dans les combinaisons discriminantes, permet en moyenne de distinguer plus de 99,5%, des sequences etudiees, quelle que soit la similarite entre ces sequences. L'utilisation d'au moins six de ces sept enzymes est suggeree afin d'assurer une bonne distinction des especes de rhizobia, connues et inconnues.

Roots and soil were sampled from around four leguminous tree species in 10-year-old plots at Bandia, Senegal. Assessments of root concentration (cm per 100 cm3 soil) and mycorrhizal colonisation (% of root length) were made and related to the abundance of spores in the soil and the above ground growth (stem diameter at 30 cm height) of the trees. Root concentrations in Acacia nilotica and Acacia tortilis plots were greater than those found in Prosopis juliflora and Acacia aneura plots at all three depths examined (0–10, 10–25, 25–50 cm). Root concentration decreased with soil depth in all plots and was greatest nearest the tree in the Acacia nilotica and Acacia tortilis plots. Mycorrhizal colonisation was highest on Prosopis juliflora roots and lowest on Acacia tortilis roots. Colonisation was not affected by distance from the tree and decreased with depth only in the Acacia aneura plot. Numbers of spores recovered from soils were generally low (27 per 100 g dry wt. soil) and were concentrated in the upper 10 cm of soil. Spores were most numerous in the Acacia aneura plot and least numerous in the Prosopis juliflora plot. Positive relationships were found between spore numbers and root concentration in the Prosopis juliflora plot and between spore numbers and mycorrhizal colonisation in the Acacia aneura plot. Of the four tree species examined, root and mycorrhizal distributions of Prosopis juliflora and Acacia nilotica showed most promise for use in agroforestry systems.

Objectif : Vigna unguiculata L. Walp. est une espece d’interet economique et ecologique au Senegal. Cependant, son rendement reste faible suite a la baisse de la fertilite des sols et de la pluviometrie. L’objectif de cette etude est de determiner l’influence des conditions pedoclimatiques et du mode cultural sur la reponse du niebe a l’inoculation avec R hizophagus irregularis. Methodologie et Resultats : L’etude a ete realisee dans deux sites a pluviometries contrastees (Diokoul et Darou Mousty). Des experimentations en culture pure de niebe ont ete realisees a Diokoul. A Darou Mousty, la culture pure a ete comparee a une culture associee au mil. Apres 30 jours de culture, les parametres de croissance et de mycorhization ont ete mesures et les teneurs en elements mineraux des sols determinees. Les resultats montrent des effets positifs significatifs (P N 0, 05%) de l’inoculation sur les parametres de mycorhization, les biomasses aerienne et racinaire des plantes et sur les teneurs en P et N des sols notamment a Diokoul (zone soudano-sahelienne). Conclusion et application : L’inoculation avec des champignons endomycorhiziens ameliore la croissance des plantes. Les resultats montrent egalement une reponse a l’inoculation en culture pure plus marquee en zone soudano-sahelienne (Diokoul) qu’en zone sahelienne (Darou Mousty). En culture associee, un effet depressif de l’inoculation sur la croissance du niebe a ete note, attestant ainsi l’influence du mode cultural sur la reponse a l’inoculation endomycorhizienne. En culture pure, l’inoculation avec R. irregularis peut etre une alternative pour ameliorer la croissance et la productivite du niebe. Mots cles : Vigna unguiculata (L.) Walp, Rhizophagus irregularis , mode cultural, conditions pedoclimatiques. Inoculation, Senegal

The growth of Frankia spp. strain ORS 020607 in BAP medium was studied by using two methods simultaneously: determination of Bradford protein content and INT (2-(p-iodophenyl-3-(p-nitrophenyl)-5-phenyl tetrazoliumchloride) reduction activity (IRA). With the latter test, red formazan crystals formed intracellularly were extracted with methanol. Colouration intensity was estimated by absorbance spectrophotometry at 490 nm. The protein content and IRA of the culture were monitored for 96 days. IRA appeared to reflect the ‘metabolically active’ biomass of Frankia more accurately than the Bradford protein estimations.

Le Senegal traverse une crise ecologique profonde due aux rigueurs climatiques, aux deficits pluviometriques repetes et surtout a des pratiques inadaptees de gestion des ressources naturelles. La superficie degradee entre 1981 et 1990 est estimee a 80 000 ha/an (Plan d’Action forestier du Senegal, 1993). Le retablissement de l’equilibre des ecosystemes naturels constitue un enjeu majeur pour ce pays dont le developpement economique et social repose en priorite sur l’utilisation du potentiel agro-sylvo-pastoral. C’est ainsi que l’option politique en matiere environnementale privilegie le reboisement comme strategie de lutte contre la desertification et la pauvrete.

Forty rhizobia nodulating four Acacia species ( A. gummifera , A. raddiana , A. cyanophylla , and A. horrida ) were isolated from different sites in Morocco. These rhizobia were compared by analyzing both the 16S rRNA gene (rDNA) and the 16S-23S rRNA spacer by PCR with restriction fragment length polymorphism (RFLP) analysis. Analysis of the length of 16S-23S spacer showed a considerable diversity within these microsymbionts, but RFLP analysis of the amplified spacer revealed no additional heterogeneity. Three clusters were identified when 16S rDNA analysis was carried out. Two of these clusters include some isolates which nodulate, nonspecifically, the four Acacia species. These clusters, A and B, fit within the Sinorhizobium lineage and are closely related to S. meliloti and S. fredii , respectively. The third cluster appeared to belong to the Agrobacterium-Rhizobium galegae phylum and is more closely related to the Agrobacterium tumefaciens species. These relations were confirmed by sequencing a representative strain from each cluster.

A collection of strains isolated from root nodules of Acacia species in Senegal was analysed previously by electrophoresis of total cell protein, auxanographic tests, rRNA-DNA hydridization, 16S rRNA gene sequencing, DNA base composition and DNA-DNA hybridization [de Lajudie, P., Willems, A., Pot, B. & 7 other authors (1994). Int J Syst Bacteriol 44, 715-733]. Strains from Acacia were shown to belong to two groups, Sinorhizobium terangae, and a so-called gel electrophoretic cluster U, which also included some reference strains from Brazil. Further taxonomic characterization of this group using the same techniques plus repetitive extragenic palindromic-PCR and nodulation tests is presented in this paper. Reference strains from Sudan and a number of new rhizobia isolated from nodules of Acacia senegal, Acacia tortilis subsp. raddiana and Prosopis juliflora in Senegal were included. As a result of this polyphasic approach, the creation of a new species, Mesorhizobium plurifarium, is proposed for a genotypically and phenotypically distinct group corresponding to the former cluster U and containing strains isolated from Acacia, Leucaena, Prosopis and Chamaecrista in West Africa (Senegal), East Africa (Sudan) and South America (Brazil). The type strain of Mesorhizobium plurifarium ORS 1032 has been deposited in the LMG collection as LMG 11892.